This invention relates generally to methods and apparatus for computed tomography (CT) imaging of moving objects, and more particularly to methods and apparatus for CT imaging of a patient""s heart.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Application of conventional computed tomography (CT) to cardiac applications has gained significant interests recently, as a result of the success of the calcification scoring and the development of phrase coded cardiac reconstruction. In both developments, the use of EKG signals is an integral part of the process, where the patient and the scanner have to be connected to an EKG monitoring device. The disadvantages of such an approach are multiple. First, the EKG device provides extra cost to the examination. Second, the attachment and detachment of EKG leads are time consuming and cumbersome. Third, it is well known that the electric signal provided by the EKG does not exactly match the mechanical motion of the heart, due to variations in the delay from patient to patient.
Therefore, it would be desirable to provide methods and apparatus to estimate cardiac motion using acquired CT projection data, but without requiring use of an EKG device.
A method for estimating motion of a part of an object, using a CT imaging system, is therefore provided in one embodiment of the present invention. This method includes steps of: scanning the object with the CT imaging system so as to acquire conjugate data samples; analyzing the conjugate data samples to remove data representative of overlapping, non-moving portions of the object; and estimating a cardiac motion from the analyzed conjugate data samples.
When the method described above is used to estimate cardiac motion, no EKG device is required for such estimation.